Currently released vehicles have a variety of electronic controllers which are configured with an in-vehicle network for mutual communication among these electronic controllers for improving user convenience and safety.
However, as in-vehicle electronic controllers have been gradually increasing in number, which are capable of interacting with various external devices, problems related to overload of an existing vehicle network and increase in costs due to wiring harnesses have been raised.
In addition, as customer demands for high-quality audio/video (A/V) data and applications using A/V data increase, bandwidth extension is necessary.
As such, a large number of vehicle manufacturers use Ethernet for in-vehicle networking and further have developed Ethernet-based around view monitoring (AVM) systems.
The Ethernet network generally includes a plurality of local area networks (LANs) and a plurality of bridges for connection among the LANs.
Ethernet includes a plurality of nodes for competitively accessing a common medium using a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol. However, a CSMA/CD scheme provides the same priority to all traffic, thus allowing the plurality of nodes to access to any traffic through competition, and thus is not appropriate for transmission of multimedia data which is sensitive to propagation time delay, e.g., A/V data.
Accordingly, Ethernet uses a technology of synchronizing timings of all network nodes so as to transmit the multimedia data.
Currently, a standard for audio/video bridging (AVB) has been developed by IEEE 802.1/1722. The AVB standard provides a quality-guaranteed transmission technology for appropriate transmission of a multimedia stream, e.g., an A/V stream, in a LAN.
As described above, according to a conventional LAN, Ethernet basically uses a frame-based packet switching technology which is not an efficient and quality-guaranteed transmission technology. To solve this problem, synchronous Ethernet, residential Ethernet, or the like have been developed by IEEE 802.2. Currently, a research on a method for implementing a similar technology on a bridge based on the conventional non-synchronous packet switching technology has been developing by IEEE 802.1/1722.
Basically, a technology for enabling synchronous traffic transmission using conventional Ethernet bridges which perform packet switching and a core thereof is to synchronize clocks of bridges within a certain geographic range. If the clocks of the bridges are synchronized, a certain-sized Ethernet frame can be accurately transmitted among the bridges with a certain time interval at a desired time. This concept is applied to a bridge mesh which can be used as an infrastructure for stably delivering synchronous traffic.
A time synchronization method among devices according to IEEE 802.1AS is a method for synchronizing a transmitter and a receiver using a time stamp including time synchronization information. In this method, a grandmaster (GM) for providing a reference time for time synchronization is selected among devices in a network, a local time of the selected grandmaster is transmitted to other devices using an announce message, and thus, the other devices use the local time of the grandmaster as a reference time. At this time, the grandmaster transmits the announce message to all other devices to transfer signal information indicating the presence of the grandmaster and a comparative value indicating suitability as a reference time providing device.
That is, the grandmaster is a top node of the IEEE 802.1/1722 time tree and periodically transmits current time information to sub nodes.
IEEE 802.1AS defines a procedure for determining a grandmaster and acquiring time synchronization, a procedure for discovering all devices on a network and controlling access to links using a plurality of control messages, and a procedure for persistently checking link states using the announce message.
However, IEEE 802.1AS cannot rapidly and efficiently acquire time synchronization when a grandmaster is lost (e.g., when the grandmaster is rebooted due to a system failure).